Mail delivery problem

We have a client that has set up a mailbox for his domain, but could not get message to go to it. I checked it further, created a test account, and the mail will no go into the test mailbox. I further found that the mail all goes to his default box. What needs to be done, so that the server recognizes that the mail is addressed to an existing mailbox? I am familiar with procmail and such, but CPanel servers do not use procmail. Any help would be appreciated.